1. Field of Invention
The present invention relates to a method for preventing choking of a urea injection nozzle on a SCR system. More particularly, the present invention relates to a method for preventing choking of a urea injection nozzle on a SCR system which determines whether a urea injection nozzle is choked and prevents choking of the urea injection nozzle in after run operation.
2. Description of Related Art
Generally, exhaust gas exhausted from an exhaust manifold of an engine is guided to a catalytic converter disposed and purified, and then passes though a muffler to reduce noise, and then finally is exhausted through an exhaust pipe to atmosphere.
The catalytic converter treats pollutant such as NOx within the exhaust gas.
A Selective Catalytic Reduction (SCR) apparatus, one of the catalytic converter, purifies NOx within the exhaust gas, and urea, ammonia, carbon monoxide and hydrocarbon (HC) and so on are used as reductant.
When the reductant is supplied to the exhaust gas, nitrogen oxide within the exhaust gas is deoxidized to nitrogen by oxidation-reduction reaction with the reductant.
FIG. 5 is a drawing showing a conventional SCR system.
As shown in FIG. 5, a conventional SCR system includes a urea tank 60 in which urea 62 is stored, a supply module 50 supplying the urea 62 within the urea tank 60, and a urea injection nozzle 20 supplying the supplied urea from the supply module 50 to a selective catalytic reduction catalyst 12.
The urea 62 passes through a filter 52 and supplied an exhaust pipe 10 in front of the SCR catalyst 12.
A dosing control unit (DCU) 25 controls supplied amount of the urea 62.
A temperature sensor 15 may detect temperature of the urea 62 or detect temperature of exhaust gas passed through the SCR catalyst 12.
In a general SCR system, urea is frozen into ice at −11° C. and corrosiveness of the urea is increased at 70° C., and thus when a vehicle or a system stops, the urea is withdrawn to the urea tank, and it is called as “after run”.
While the SCR apparatus is operated, the urea is supplied from the urea tank 60 and the supply module 50 to the urea injection nozzle 20, however in an after run operation, the urea is withdrawn in reverse order.
If the urea injection nozzle where the urea contacts is applied heat by exhaust gas temperature or coolant, the temperature of the urea reaches to temperature (71-75° C.) of crystallization and the after run is operated with 100% open of the urea injection nozzle, crystallization of the urea is generated in the urea injection nozzle and thus the urea injection nozzle may be choked.
That is, while the after run is operated, urea the injection nozzle 20 has to be opened, and crystallization of the urea is generated in the urea injection nozzle, however choking of the urea injection nozzle 20 may not be detected.
Particularly, if flowing backward of the urea occurs in winter season, the system may be damaged.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.